1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a resonator, which extracts an oscillation or wave of a predetermined frequency using a resonance phenomenon, and a fabrication method thereof.
2. Description of the Related Art
Recently, with a rapid development of mobile communication apparatuses, chemical and bio apparatuses, etc., a demand for small and lightweight filters, oscillators, resonant elements, acoustic resonant mass sensors, etc., which are used in such apparatuses, is increasing.
As a means for realizing the small and lightweight filter, the oscillator, the resonant element, the acoustic resonant mass sensor, etc, a film bulk acoustic resonator (FBAR) is known. The FBAR is advantageous in that it can be manufactured in large quantities at a minimal cost and embodied in a micro-size. In addition, the FBAR can realize a high value of quality factor Q, which is a main characteristic of the filter, and can be used even in a micro frequency band, particularly, a personal communication system (PCS) band and a digital coreless system (DCS) band.
In general, the FBAR is configured in a structure including a resonating unit in which a first electrode, a piezoelectric film, and a second electrode are layered in turn on a substrate.
An operation of the FBAR is as follows. First, when electric energy is applied to the first and the second electrodes, an electric field is incited in the piezoelectric film. The electric field incites a piezoelectric phenomenon to the piezoelectric film, so that the resonating unit oscillates in a predetermined direction. As a result, a bulk acoustic wave is produced in the same direction as the oscillating direction of the resonating unit to cause resonances.
However, the FBAR generates an insertion loss in a real operation. Such an insertion loss is generated because when the FBAR generates the resonances, mechanical energy is changed into thermal energy, or acoustic energy in the form of the bulk acoustic wave converted into a lateral mode form is leaked outside an active region of the resonating unit or into the substrate.
To reduce the insertion loss, a resonator has been proposed in which a frame-like layer with a narrow width is formed along an edge of an active region that causes resonances. The resonator is disclosed in U.S. Pat. No. 6,812,619. The frame-like layer causes an edge part and a center part of an active region portion of an upper electrode to have heights different from each other. Accordingly, in a resonance, a frequency difference exists between an edge part and a center part of an active region of the resonator due to a difference in height between the edge part and the center part of the active region portion of the upper electrode on which the frame layer is formed. As a result, a leakage of a bulk acoustic wave converted into a lateral mode form outside the active region is prevented, and thus the center part of the active region causes uniform resonances. However, the resonator is disadvantageous in that since the frame-like layer is formed to change the height of the edge part of the active region portion of the upper electrode, resonance characteristic of the edge part of the active region portion, particularly, a value of an effective piezoelectric coefficient Kt2 is changed.
Also, the insertion loss of the FBAR is influenced by electric resistance according to electric conductivity of the electrodes forming the FBAR. Accordingly, to reduce the insertion loss caused by the electric conductivity of the electrodes, it is preferable to increase the electrodes in thickness or to form the electrodes with a material having a high electric conductivity. However, there is a limit, in that such a method increases fabrication costs. Therefore, there is required a new method capable of minimizing the insertion loss of the FBAR caused by the electric conductivity of the electrodes without increasing the electrodes in thickness or forming the electrodes with the costly material having the high electric conductivity.